1. Technical Field of the Invention
This invention relates to optical inspection systems. More particularly, and not by way of limitation, the present invention is directed to the automated optical inspection and analysis of regularly-patterned production surfaces such as those on semiconductor wafers, using multi-wavelength, narrow-bandwidth imaging.
2. Description of Related Art
Optical inspection of articles of manufacture, either finished or in-process, may range from simple visual inspection to sophisticated computer-assisted inspection. Automated inspection is increasingly valuable as equipment and techniques improve because it is fast, reliable, and can frequently detect production defects that cannot be easily perceived by the unaided human eye.
This is true in the case of the in-process inspection of semiconductor wafers. Semiconductor wafers are manufactured in stages, with each stage representing the development of a new layer, or set of surface structures that form a functional part of the electronic devices that will populate the wafer when it is finished. The structures of each stage are formed by selectively etching away or treating carefully selected areas of the surface. The selection of areas to be etched or treated is often accomplished by covering the remaining area with a protective material called photoresist.
The photoresist is first applied evenly to the entire wafer surface, then selectively exposed to light emitted through a mask. This changes the nature of the exposed area so that it becomes, for example, more or less soluble. Then during development, the exposed areas are either retained or washed away (depending on the type of photoresist used), leaving a pattern of resist structures that will protect the wafer surface under them as the remainder of the surface is altered. During the process of etching, for example, unprotected areas are removed to a certain depth, perhaps to be filled later or otherwise treated. The protective photoresist is then removed, leaving only the desired surface configuration. The next stage can then be prepared for treatment and the process repeated until the desired surface structures have been completely formed.
Frequent inspections of the wafer surface are desirable during the production process, especially at the point where photoresist structures have been formed. Although many types of defects can be repaired, the photoresist is relatively easily removed and reapplied, so it is most advantageous to detect defects in it, rather than etching an improperly treated wafer that would be more difficult and expensive to repair.
Wafers in the process of manufacture can, of course, and sometimes are visually inspected for defects. Generally, however, an automated inspection system is used. In such systems, some form of electromagnetic energy, often but not always visible light, is directed at the surface to be inspected. The image created by the light reflecting from the surface is then captured and translated into digital form for processing by a computer.
The surface-image data may, for example, be analyzed to determine if unusual or tell-tale patterns are present that are commonly associated with certain kinds of defects. In one such technique, called image decomposition, surface structures are traced and described in terms of image grammars composed of units called primitives. One such technique is explained in detail in co-owned and co-pending U.S. patent application Ser. No. 09/074,301, entitled SYSTEM AND METHOD OF OPTICALLY INSPECTING MANUFACTURED DEVICES, filed May 6, 1998, a continuation-in-part of U.S. patent application Ser. No. 08/867,156, which issued on Jul. 18, 2000 as U.S. Pat. No. 6,091,846, entitled METHOD AND SYSTEM FOR ANOMALY DETECTION, both of which are incorporated herein by reference in their entirety. In some systems, the images associated with each inspection are classified, stored, and indexed for later use. Comparisons may be made to detect errors in the defect-detection process itself and to analyze the manufacturing process in order to determine, if possible, the root cause of frequently discovered defects in the hope of minimizing the occurrence of similar defects in the future.
In some instances, capturing an image of light reflected specularly from the wafer surface is inadequate for efficient and comprehensive defect detection. It has been found, for example, that defects such as focus offset (defocus errors) due to the presence of stray particles, errors in wafer development, etching or stripping, or to insufficient developer, are sometimes detectable by examining the light diffracted from the structures on the production surface. However, some defocus errors are so small as to require a high resolution imaging capability, and existing systems do not detect such errors.
When, as is the case with a properly-constructed semiconductor wafer, an object's surface features are small and sufficiently uniform so as to form a regular pattern that amounts to or approximates a diffraction (or, more properly, a reflection) grating, an analysis of the diffracted light is also useful. One method of using diffracted light is disclosed in U.S. Pat. No. 5,777,729 to Aiyer et al. Aiyer uses an elongated and extended monochromatic light source to illuminate an entire wafer surface, with each point thereon being illuminated by light at different angles. A diffraction efficiency is then calculated and utilized for defect detection. Other methods of using diffracted light are disclosed in co-owned and co-pending U.S. patent application Ser. No. 10/094,119, filed Mar. 8, 2002, entitled SYSTEM AND METHOD FOR PERFORMING OPTICAL INSPECTION UTILIZING DEFRACTED LIGHT (claiming priority to U.S. Provisional Patent Application No. 60/278,961 entitled, METHOD OF PERFORMING OPTICAL INSPECTION, filed Mar. 27, 2001), and co-owned and co-pending U.S. patent application Ser. No. 10/298,391, filed Nov. 18, 2002, entitled OPTICAL INSPECTION METHOD UTILIZING ULTRAVIOLET LIGHT, both of which are incorporated by reference herein in their entirety.
The utilization of diffracted light, however, somewhat complicates the inspection process. For example, when monochromatic light is directed at a known angle of incidence at a particular area on the wafer surface for which the grating pitch (i.e., distance between the regular surface features) is known, it is possible to predict the angle of first- (or other-) order diffraction, since the angle(s) of diffraction are a function of the grating pitch and the angle of incidence. For a light source in a fixed position, the camera or other image-capturing device used must be repositioned each time the grating pitch changes in order to capture light exiting the surface at a particular order of diffraction. Additionally, for a fixed light source and fixed grating pitch, the camera must be repositioned to capture light exiting the surface at different orders of diffraction. Finally, if the wavelength of the incident light is changed, once again, the camera must be repositioned to capture light exiting the surface at any particular order of diffraction.
Another problem encountered in diffractive imaging is that some types of surface defects are more readily detected at a first range of wavelengths while other types of surface defects are more readily detected at a second range of wavelengths. Thus, monochromatic illumination at a particular wavelength may not provide the sensitivity required to detect defects that are more readily detectable at other wavelengths. Therefore, broad bandwidth illumination is required in order to provide imaging at all of the required wavelengths. The problem with this approach is that broad bandwidth illumination, in diffractive imaging systems, degrades both system contrast and sensitivity for detecting defocus errors.
In order to overcome the disadvantage of existing solutions, it would be advantageous to have a system and method for multi-wavelength, narrow-bandwidth detection and analysis of surface defects that does not degrade system contrast or defocus error detection sensitivity. The present invention provides such a system and method.